1. Technical Field
The invention relates generally to backlight modules and, more particularly, to an edge-lighting backlight module for providing a planar illuminating light to a liquid crystal display (LCD) device.
2. Discussion of Related Art
With the extensive application of liquid crystal displays (LCDs) in electronic display devices, the requirement for effective and efficient liquid crystal display devices increases rapidly. In a liquid crystal display device, a liquid crystal is a substance that does not itself radiate light. Instead, the liquid crystal relies on receiving light from a light source to thereby display images and/or data. In the case of a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light. A conventional backlight module can be divided into two types, i.e., a direct type and an edge type, according to the location of the light sources. In an edge-type backlight module, the light sources are located facing the incident surface of light guide plate. Such edge-type backlight modules are widely used in LCD devices. Light beams emitted from the light sources are optically coupled into the incident surface, enter the light guide plate, advantageously reflected, as needed, by the microstructure of the back reflective surface, and then transmitted out from the emitting surface uniformly to illuminate an LCD panel.
FIG. 8 (Prior art) represents a conventional edge-lighting type backlight module 80. The backlight module 80 includes a light source 810, a reflective plate 820, a light guide plate 830, a diffusion sheet 840, and a prism sheet 850. The light source 810 is positioned adjacent an incident surface of the light guide plate 830. The reflective plate 820 is located below the light guide plate 830 and is configured for reflecting light beams that is emitted from a bottom surface of the light guide plate 830 into the light guide plate 830. The diffusion sheet 840 is located above the light guide plate 830 and is configured for uniformly diffusing the emitted light beams. The prism sheet 850 is positioned above the diffusion sheet 84 and is configured for collimating the emitted light beams, thereby improving the brightness of illumination. The backlight module 830 can use, e.g., cold cathode fluorescent lamps (CCFL) or light emitting diodes (LED) as the light source 810.
A small-sized backlight module usually uses at least one LED as a light source and a large-sized backlight module usually uses a CCFL as a light source. Advantages of LED usage over CCFL usage include the following. Firstly, the LED has a long life, a bright color, and a high reliability. Secondly, the LED is not harmful to the environment, while the CCFL potentially is because of the presence of mercury in the fluorescence tube thereof. So, it may be a development trend that LEDs are used as the preferred light source of edge-type backlight modules. However, referring to FIG. 9, when LEDs are used in the backlight module as the light source 810, a number of bright areas 832 may be occur in areas adjacent to the light source 810, and a number of dark areas 834 may appear between neighboring bright areas 832. Therefore, a light column phenomenon formed by the bright areas 832 and the dark areas 834 can occur due to the restriction of a light emitting angle of LEDs. This phenomenon reduces light distribution uniformity.
Referring to FIG. 10, another conventional backlight module 90 is shown. The backlight module 90 includes a plurality of LEDs 910 and a light guide plate 930. The light guide plate 930 includes an incident surface 932 facing the LEDs 910, an emitting surface 934 adjoining the incident surface 932, and a plurality of light diffusing portions 936 defined in the incident surface 932, spatially corresponding to the LEDs 910. The light diffusing portions 936 has a plurality of V-shaped grooves arranged regularly and periodically in a direction perpendicular to the emitting surface 934. Light beams incident on the light diffusing portions 936 are scattered. The diffusing portions 936 can thereby reduce the area of dark areas formed adjacent the incident surface 932.
FIG. 11 shows that a light beam is refracted into the above-described light guide plate 930 through the incident surface 932 having a plurality of diffusing portion 936. According to the Fresnel formula of reflection and deflection, a deflection angle β can be defined by the following equation:
      β    =          90      -              α        2            -              arcsin        ⁢                  (                                    sin              ⁡                              (                                  90                  -                                      α                    2                                                  )                                      n                    )                      ,wherein α is the vertex angle of V-shaped grooves of the diffusing portion 936, and n is a refractive index of the light guide plate 90. The deflection angle β cannot be greater than or even equal to 90 degrees, according to the equation. For example, if the light guide plate 90 is formed of polymethyl methacrylate (PMMA), the largest deflection angle β is generally smaller than 50 degrees. Therefore, the diffusing portions 936 cannot completely eliminate dark areas formed adjacent the incident surface 932. In addition, some of the light beams can be reflected at the incident surface 932 of the light guide plate 930, thus a utilization efficiency of light energy of the backlight module 90 is decreased.
Referring to FIG. 12, a still another conventional backlight module 100 is shown. The backlight module 100 is similar to the backlight module 90, except that light diffusing portions 136 thereof are different from the diffusing portion 936 of the backlight module 90. The backlight module 100 includes a light guide plate 130 having an incident surface 132, an emitting surface 134, and a plurality of light diffusing portions 136. The light diffusing portions 136 are, particularly, a plurality of grooves defined in the incident surface 132. The diffusing portions 136 can also reduce the size of the dark areas formed adjacent the incident surface 132. However, similarly to the above described light guide plate 93, the diffusing portions 136 still cannot completely eliminate dark areas formed adjacent the incident surface 132. Some of the light beams can be reflected at the incident surface 132 of the light guide plate 130, thus a utilization efficiency of light energy of the backlight module 100 is decreased.
Referring to FIG. 13, a further another conventional backlight module 120 is shown. The backlight module 120 includes a plurality of LEDs 142, a light guide plate 140, and a reflector 144. The light guide plate 140 includes an incident surface 148, which faces the LEDs 142, and an emitting surface 146 adjoining the incident surface 148. The reflector 144 has a plurality of curved sections, each partly surrounding the respective LED 142. Each of the LEDs 142 has a luminescent surface 150 that faces the adjacent curved section of the reflector 144. Light beams, emitted from the LEDs 142, are redirected by the reflector 144 and enter into the light guide plate 140 through the incident surface 148 thereof However, some of the light beams are blocked by the respective LEDs 142, thereby preventing the light beams from reaching the incident surface 148 adjacent to the respective LEDs 142. As a result, a plurality of dark areas is formed in the light guide plate 140, adjacent the incident surface 148.
What is needed, therefore, is a backlight module which can completely eliminate the dark areas formed adjacent the incident surface of the light guide plate and is capable of improving a uniformity of illumination and a utilization efficiency of light energy.